The present invention relates to a shift control system for a continuously variable transmission (CVT) and particularly to a starting shift (i.e., gear shift at start of a vehicle) control system for a CVT when the vehicle is started by starting an engine, with the CVT being in a high speed side gear ratio due to traction of the vehicle under an engine stopped condition where an oil pressure for shift control is not produced. The present invention further relates to a starting shift control method for a continuously variable transmission.
A toroidal or V-belt CVT is capable of producing a continuously variable output which is obtained by varying the rotational speed of an input from an engine continuously and is therefore excellent in transmission quality as compared with a transmission having limited gear ratios.
Normal shift of the CVT is made as follows. Pulse gears are provided which are rotatable with an input shaft and output shaft of the CVT, respectively. Magnetic pickups or hole IC sensors are disposed opposite to the respective pulse gears for fetching input rotation pulses generated at repetition periods corresponding to input rotation and output rotation pulses generated at repetition periods corresponding to output rotation. On the basis of the input and output rotation pulses, an actual gear ratio is obtained. The CVT is shifted so that the actual gear ratio becomes equal to a target gear ratio.
How to obtain the actual gear ratio will be described with respect to the case where the periods of input rotation pulses and the periods of output rotation pulses are caused to become shorter gradually with increase of the input rotation speed and the output rotation speed. Measurement of times t0, t1, t2xe2x80x94for two periods of output rotation pulses and T1, T2, T3xe2x80x94for two periods of input rotation pulses is made continuously. At the end of each measurement of two periods of the input and output rotation pulses, the time for two periods of output rotation pulses which was obtained by the previous measurement is divided by the time for two periods of the input rotation pulses which was obtained by the previous measurement, to obtain the actual gear ratios (T0/t0), (T0/t1), (T1/t1), (T1/T2) as shown in FIG. 5.
When the CVT is in the normally used speed range, the repetition periods of the input and output rotation pulses are relatively short, so that the actual gear ratio can be obtained with a high accuracy on the basis of the repetition periods of the input and output rotation pulses.
However, since the input and output shafts differ in normally used speed range from each other, the input and output pulse gears are constructed so as to have a different number of the gear teeth. By this, the timing at which the input rotation pulse repetition period is measured and the timing at which the output rotation pulse repetition period is measured (i.e., timings at which data are updated) are different from each other. Further, since a filter processing is conducted for removal of noise, accurate processing of the actual gear ratio cannot be attained when updating of data is conducted once or so. For the above reason, in the quite low speed range immediately after start of the vehicle, the input and output rotation repetition periods exceed beyond the limits, so that as a matter of fact, the actual gear ratios cannot be calculated.
In this instance, it is a usual practice to set the actual gear ratio at an allowable fixed value (normally, lowermost speed gear ratio) for the convenience of a shift control.
On the other hand, when the CVT, particularly toroidal CVT receives an input of rotational force reversely from the wheel side due to traction of the vehicle under an engine stopped condition where an oil pressure for a shift control is not produced, it tends to shift to a high speed side gear ratio arbitrarily. When the engine is started with the CVT in a high speed side gear ratio, the following problem is caused.
Although it is natural that the CVT determines the target gear ratio to a lowermost speed gear ratio at start of the vehicle, a shift from the above described high speed side gear ratio before start of the vehicle to the lowermost speed gear ratio can not be attained unless rotation of the CVT is caused by the start of the vehicle, thus causing the vehicle to start with the CVT in the above described high speed side gear ratio (this start will be referred to hereinafter as high start).
At the time of this high start, the driver feels a lack of torque due to the high speed side gear ratio and depresses further the accelerator pedal having been depressed at start of the vehicle. By this, the vehicle is started to cause rotation of the CVT. Immediately after rotation of the GVT is caused, a shift from the high speed side gear ratio before start to the lowermost speed gear ratio is caused. Such a shift at start of the vehicle is undesired since it inevitably deteriorates the riding comfort of the vehicle.
This problem is pronounced particularly in the toroidal CVT since the shift response of the toroidal CVT is particularly faster as compared with the other type of CVT.
In the meantime, when the CVT is caused to operate at such a rotation speed that enables calculation of the actual gear ratio to be executed accurately in the above described manner, it becomes possible to execute a normal shift control for causing the actual gear ratio to follow the target gear ratio (i.e., the lowermost speed gear ratio due to start) which is determined in accordance with the vehicle driving condition. However, this is the time after the above described problem has been already caused and settled.
To solve the above described problem, it is considered such a countermeasure against the high start wherein the target gear ratio is once regulated not to the lowermost speed gear ratio but to the intermediate gear ratio, so that the transmission is once shifted from the high speed side gear ratio before start down to the intermediate gear ratio and thereafter to the lowermost speed gear ratio which is a normal target gear ratio at start of the vehicle.
However, when the actual gear ratio is calculated from the input and output pulse repetition periods as in the prior art, it is impossible, as described above, to calculate the actual gear ratio when the CVT is in a quite low speed range after start of the vehicle, thus making it impossible to detect even the high start and therefore making it possible to employ the above described countermeasure.
It is accordingly an object of the present invention to provide a starting shift control system for a continuously variable transmission which is free from the above described problem inherent in the prior art device.
It is a further object of the present invention to provide a starting shift control method for a continuously variable transmission which can solve the above noted problems.
To accomplish the above objects, there is provided according to an aspect of the present invention a starting shift control system for a continuously variable transmission for a vehicle, comprising an input rotation detector for detecting an input rotation of the transmission drivingly connected to a prime mover of the vehicle and producing input rotation pulses at periods corresponding to the input rotation, an output rotation detector for detecting an output rotation of the transmission and producing output rotation pulses at periods corresponding to the output rotation, and a control unit for calculating an actual gear ratio on the basis of the input rotation pulses and the output rotation pulses and controlling so that the actual gear ratio becomes equal to a target gear ratio, wherein the control unit includes a high start judgment device for judging whether or not a start of the vehicle is a high start in a high speed side gear ratio by comparison between the number of the input rotation pulses and the number of the output rotation pulses from the time a first vehicle starting operation is made after start of the prime mover, and a high start control device for restricting downshifting from the high speed side gear ratio to the target gear ratio when the start of the vehicle is judged to be the high start.
According to another aspect of the present invention, there is provided a starting shift control method for a continuously variable transmission for a vehicle, comprising detecting an input rotation of the transmission drivingly connected to a prime mover of the vehicle and producing input rotation pulses at periods corresponding to the input rotation, detecting an output rotation of the transmission and producing output rotation pulses at periods corresponding to the output rotation, and calculating an actual gear ratio on the basis of the input rotation pulse and the output rotation pulse, and controlling so that the actual gear ratio becomes equal to a target gear ratio, wherein the controlling includes judging whether or not a start of the vehicle is a high start in a high speed side gear ratio by comparison between the number of the input rotation pulses and the number of the output rotation pulses from the time a first vehicle starting operation is made after start of the prime mover, and restricting downshifting from the high speed side gear ratio to the target gear ratio when the start of the vehicle is judged to be the high start.